feat(docker): implement full frontend Next.js containerization and multi-service docker-compose

- Create multi-stage standalone Dockerfile for Next.js frontend (node:20-alpine, non-root nextjs:nodejs user)
- Configure output: 'standalone' in refer_landing_page/next.config.js
- Add refer_landing_page/.dockerignore for build context optimization
- Wire anl-backend and anl-frontend services in root docker-compose.yml with healthchecks, network dependencies, and persistent volume storage

refer_landing_page/next.config.js

@@ -1,6 +1,7 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   reactStrictMode: true,
+  output: "standalone",
   async redirects() {
     return [
       {